Receiver, transmitter and communication control program

ABSTRACT

A transmitter has a network interface unit connected to a wireless network capable of transmitting contents for which copyright protection is necessary, an encryption processing unit configured to encrypt contents for which copyright protection is necessary, an RTT measuring unit configured to measure a round trip time after a predetermined packet is transmitted to a receiver, until a response corresponding to the transmitted packet is received, a communication permission determination unit configured to permit transmission of the contents for which copyright protection is necessary when the round trip time measured by the RTT measuring unit is within a predetermined time, and a parameter modification unit configured to change parameters of the wireless network before and/or after the RTT measuring unit performs the measurement of the round trip time.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a receiver, a transmitter and acommunication control program which communicate contents for whichcopyright protection is necessary via a wireless network.

2. Related Art

Products called as digital information consumer electronics haveincreased. It is predicted that these products will be widespreadaccording to the beginning of contents distribution on the Internet. Ina category of the digital information consumer electronics, there areincluded various kinds of products which deal with digital data anddigital contents such as a mobile audio player, a digital broadcastingTV, a set top box and a hard disk recorder.

One of problems to be taken into consideration in accordance with spreadof these products is copyright protection of contents. Digital data hasan advantage in that quality does not deteriorate even if the contentsare copied, but has a disadvantage in that illegal copy is easilyconducted and copyright protection becomes insufficient.

Because of this, IEEE1394, which is a digital network connecting betweendigital AV equipments, has an authentication & key exchange function anda data encryption function.

Here, it is assumed to transmit AV data that copyright protection isnecessary from a certain transmitter to a receiver. In this case, wehave to pay attention that copyright protection relates to transmissionof AV data only in a range that each individual person (or in a case ofextended interpretation, each family member) enjoys. Transmission andreception of AV data between different persons should not be permitted,as long as permission of a contents provider is not obtained.

As one solution of protecting copyright on a network, there has beenknown DTCP (Digital Transmission Content Protection).

The DTCP is a copyright protection scheme which has become a de factostandard in IEEE1394, USB, IP and so on. The DTCP performsauthentication & key exchange processing between the transmitter and thereceiver, to transmit the encrypted AV data (see http://www.dtla.com).

Generally, in order to protect copyright in a transmission system, theAV data is transmitted by the following processing procedure. A commandfor transmitting and receiving AV data is issued between the transmitterand the receiver. For example, the receiver issues a reproductioncommand as one of AV control commands to the transmitter.

Subsequently, transmission of AV data from the transmitter to thereceiver is begun after AV data is encrypted to protect copyright.Before or after the transmission of AV data, authentication & keyexchange processing is performed to protect copyright between thetransmitter and the receiver.

When the authentication & key exchange processing is successful, thetransmitter and the receiver can share an encryption key of AV data, orcalculate the encryption key by themselves. The receiver decrypts theencrypted AV data to reproduce the decrypted AV data.

Procedure of the DTCP is standardized as “DTCP over Bluetooth” even inBluetooth included in a close range wireless standard. By using theDTCP, it is possible to securely transmit AV data that copyrightprotection is necessary.

However, progress of recent Internet technology causes new problems. Inthe Internet, it is possible to transmit and receive arbitrary databetween users away an arbitrary distance.

Therefore, it is possible to easily transmit data between Japan and theUnited States at low cost.

It is possible to encrypt AV data that copyright protection is necessaryfor each packet. Also it is technically possible to transmit theencrypted packet in a capsulated state on Internet. Accordingly, thereis a likelihood that a packet of AV data transmitted by the Bluetoothdevice which can communicate at only a close range may be converted intoan internet capsule to transmit it to a distant location. In this case,if both of the receiver for receiving the internet packet and theBluetooth device at sending side support the DTCP function, theauthentication & key exchange processing is established between both ofthe receiver and the Bluetooth device, and it is possible to record andreproduce the AV data.

The “DTCP over IP” which prescribes procedure of copyright protectionusing the internet protocol (IP) adopts an RTT (Round Trip Time) as atechnique for performing inside-home communication. More specifically,before or after the authentication & key exchange processing forcopyright protection, or along the way of the authentication & keyexchange process, the RTT is measured. The RTT is a time period from atime when a specific packet is sent from the transmitter to thereceiver, until a time when the specific packet is sent back from thereceiver to the transmitter. At this time, the receiver which receivesthe specific packet may transmit the specific packet as it is, or mayinclude new data in the received specific packet. In this way, if thetransmitter can correctly acknowledge a relationship between thetransmitted original packet and the received packet, there is noparticular limitation to a format of the packet sent back by thereceiver. The transmitter measures the RTT, and if the measured value isequal to or less than a constant value, the transmitter can determinethat the receiver is located in close range, i.e. the receiver islocated inside the home. Therefore, it is possible to preventtransmission of AV data at a long distance.

SUMMARY OF THE INVENTION

The present invention provides a receiver, a transmitter and acommunication control program which can surely limit a transmissionrange of contents to protect copyright of contents when contents aretransmitted by using a wireless network.

According to one embodiment of the present invention, a transmitter,comprising:

a network interface unit connected to a wireless network capable oftransmitting contents for which copyright protection is necessary;

an encryption processing unit configured to encrypt contents for whichcopyright protection is necessary;

an RTT measuring unit configured to measure a round trip time after apredetermined packet is transmitted to a receiver, until a responsecorresponding to the transmitted packet is received;

a communication permission determination unit configured to permittransmission of the contents for which copyright protection is necessarywhen the round trip time measured by the RTT measuring unit is within apredetermined time; and

a parameter modification unit configured to change parameters of thewireless network before and/or after the RTT measuring unit performs themeasurement of the round trip time.

According to one embodiment of the present invention, a receiver,comprising:

a network interface unit connected to a wireless network capable ofreceiving contents for which copyright protection is necessary;

an encryption processing unit configured to decrypt contents for whichcopyright protection is necessary;

an RTT measuring unit configured to measure a round trip time after apredetermined packet is transmitted to a transmitter, until a responsecorresponding to the transmitted packet is received;

a communication permission determination unit configured to permitreception of contents for which copyright protection is necessary whenthe round trip time measured by the RTT measuring unit is within apredetermined time; and

a parameter modification unit configured to change parameters of thewireless network before and/or after the RTT measuring unit performs themeasurement.

According to one embodiment of the present invention, a communicationcontrol program, comprising:

measuring a round trip time after a predetermined packet is transmittedto the other communication apparatus, until a response corresponding tothe transmitted packet is received;

permitting transmission or reception of contents for which copyrightprotection is necessary when the measured round trip time is within apredetermined time;

transmitting or receiving the encrypted contents via a wireless networkwhen transmission or reception of the contents is permitted; and

changing parameters of the wireless network before and/or after theround trip time is measured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a connection relationship between atransmitter and a receiver according to a first embodiment.

FIG. 2 is a block diagram showing one example of internal configurationof the transmitter 1 in FIG. 1.

FIG. 3 is a block diagram showing one example of internal configurationof the receiver in FIG. 1.

FIG. 4 is a sequence diagram showing a detailed procedure of the RTTmeasuring units 17 and 27.

FIG. 5 is a flowchart showing one example of the RTT measurementprocessing performed by the RTT measuring unit 17 in the transmitter 1.

FIG. 6 is a diagram showing a connection relationship between the master30 and the slaves 31.

FIG. 7 is a diagram explaining the sniff mode.

FIG. 8 is a diagram explaining a polling interval.

FIG. 9 is a sequence diagram showing a first example of a timing ofchanging the Bluetooth parameters.

FIG. 10 is a sequence diagram showing a second example of a timing ofchanging the Bluetooth parameters.

FIG. 11 is a sequence diagram showing a third example of a timing ofchanging the Bluetooth parameters.

DETAILED DESCRIPTION OF THE INVENTION

Hereafter, a receiver and a receiving method according to the presentinvention will be described more specifically with reference to thedrawings.

(First Embodiment)

FIG. 1 is a diagram showing a connection relationship between atransmitter and a receiver according to a first embodiment. As shown inFIG. 1, the transmitter 1 and the receiver 2 perform a close rangewireless communication via a wireless network 3 based on a Bluetoothstandard.

Each of the transmitter 1 and the receiver 2 implements a Bluetooth AVprofile and “DTCP over Bluetooth” which prescribes procedure ofcopyright protection for Bluetooth. Here, the Bluetooth AV profile is astipulation for transmitting audio and video on Bluetooth and forperforming command control. The “DTCP over Bluetooth” implements an RTTmeasurement scheme for performing inside-home communication.

FIG. 2 is a block diagram showing one example of internal configurationof the transmitter 1 in FIG. 1. FIG. 3 is a block diagram showing oneexample of internal configuration of the receiver in FIG. 1. Thetransmitter 1 of FIG. 2 has a DTCP device ID recording unit 10, a DTCPauthentication & key exchange processing unit 11, a contents providingunit 12, an encryption processing unit 13, a packet processing unit 14,a communication processing unit 15 and a Bluetooth interface unit 16.The receiver 2 of FIG. 3 has a DTCP device recording unit 20, a DTCPauthentication & key exchange processing unit 21, a contents processingunit 22, an encryption processing unit 23, a packet processing unit 24,a communication processing unit 25, and a Bluetooth interface unit 26.

The DTCP device ID register units 10 and 20 record a DTCP device IDexpressing a unique ID which is provided by a DTCP certificationorganization and included in its own certificate, an ID expressing itsown Bluetooth address or an ID expressing a communication partner suchas a Bluetooth address of the communication partner. The DTCPauthentication & key exchange processing units 11 and 21 perform theauthentication & key exchange processing with the other communicationapparatus corresponding to the communication partner, hereinafter calledas a communication partner apparatus. The DTCP authentication & keyexchange processing units 11 and 21 have RTT measuring units 17 and 27which perform measurement of RTT, respectively.

The contents processing unit 22 performs processing for extractingcontents from the received data. The contents providing unit 12 acquiresthe contents to be transmitted and provides the acquired contents to theencryption processing unit 13. The encryption processing units 13 and 23perform encryption or decryption of the packets. The packet processingunits 14 and 24 perform generation or decomposition of the packets. Thecommunication processing units 15 and 25 perform modulation/demodulationprocessing, error correction processing, base band processing and so onof the transmitted and received packets. The Bluetooth interface units16 and 26 set various parameters of Bluetooth to send out communicationdata to the wireless network 3 and to receive the packets from thewireless network 3.

FIG. 4 is a sequence diagram showing a detailed procedure of the RTTmeasuring units 17 and 27. Hereinafter, an example in which thetransmitter 1 performs the RTT measurement processing will be described.Contrary, the receiver 2 may perform the RTT measurement processing ofthe transmitter 1. In this case, if the RTT measurement is successful,the receiver 2 notifies the transmitter of success of measurement.

First of all, the transmitter 1 transmits and receives the command forthe authentication & key exchange processing with the receiver 2 (stepS1). Subsequently, a preliminary preparation for the authentication &key exchange processing is performed (step S2). Here, certificates andrandom numbers are transmitted and received to/from each other to verifythe respective certificates. The certificates include descriptions as towhether the transmitter 1 and the receiver 2 have the RTT measurementfunction. Accordingly, by referring to the certificate transmitted fromthe receiver 2, it is possible to detect whether or not the receiver 2has the RTT measurement function.

When the receiver 2 has the RTT measurement function, the RTTmeasurement is performed. If the receiver 2 has been registered to theDTCP device ID register unit 10 in the transmitter 1, it is determinedthat the RTT has been already calculated, and the RTT measurement isomitted.

When the processing of step S2 is completed, parameters used forgenerating the key in the authentication & key exchange processing areexchanged (step S3). The parameters are values of first phase ofDiffie-Hellman using an elliptic curve. Subsequently, a preliminarypreparation processing for the RTT measurement is performed (step S4).Here, RTT_Ready commands indicating that preparation for the RTTmeasurement is ready are exchanged to each other.

Subsequently, the transmitter 1 transmits RTT_Setup command informingthe receiver 2 of the beginning of the RTT measurement, and the receiver2 sends back the response for the RTT_Setup command to the transmitter 1(step S5).

Subsequently, the RTT measurement is begun, and the transmitter 1transmits RTT_Test command to the receiver 2. The receiver 2 which hasreceived the RTT_Test command responds at once (step S6). Thetransmitter 1 measures a time period from a time of transmitting theRTT_Test command until at time of receiving the response. Subsequently,the transmitter 1 transmits RTT_Verify command to the same receiver 2 toverify whether or not the RTT measurement has been performed, and thereceiver 2 sends back the response (step S7).

Subsequently, the transmitter 1 and the receiver 2 perform the remainingprocessing of the authentication & key exchange processing suspendedalong the way (step S8).

As a result of performing the RTT measurement in step S6, if the RTT isequal to or less than a predetermined value, it is determined that thereceiver 2 is located inside the home, i.e. near the transmitter 1, andthe transmitter 1 registers a device ID of the receiver 2 to the DTCPdevice ID recording unit 20. The registered device ID may be invalidatedafter M hours have been passed. Therefore, there is no likelihood thatthe registered device ID is illegally acquired by some means to makeinappropriate use. Furthermore, due to radio wave status, trouble of thereceiver 2 and so on, there is a possibility that the RTT measurement isnot temporarily performed correctly. Therefore, the RTT measurement maybe repeatedly performed multiple numbers of times.

FIG. 5 is a flowchart showing one example of the RTT measurementprocessing performed by the RTT measuring unit 17 in the transmitter 1.First of all, the authentication & key exchange processing is begun(step S11). Subsequently, it is determined that the receiver 2 has theRTT measurement function (step S12). When the receiver 2 has the RTTmeasurement function, it is determined whether or not the device ID ofthe receiver 2 is registered in the DTCP device ID register unit 10(step S13).

When it is determined that the receiver has not yet been registered instep S13, the RTT measurement is performed (step S14). A detailedprocedure of the RTT measurement will be described later.

After performing the RTT measurement, it is determined whether or notthe RTT is equal to or less than a predetermined value (step S15). Ifthe RTT is larger than the predetermined value, it is determined whetheror not measurement counts are equal to or less than N (step S16). If themeasurement counts are equal to or less than N, the processing returnsto step 514 to perform the RTT measurement again. If the RTT does notbecome equal to or less than the predetermined value even if the RTTmeasurement is performed N times, a suspending process the RTTmeasurement is performed (step S17).

If it is determined that the RTT is equal to or less than thepredetermined value in step S15, the device ID of the receiver 2 isregistered to the DTCP device ID recording unit 10. A timer measurementis begun (step S18). When a measuring time by the timer measurementreaches a predetermined time M, the registered device ID isautomatically deleted from the DTCP device ID recording unit 20.

When it is determined in step S12 that the receiver 2 does not have theRTT measurement function, when it is determined in step S13 that thereceiver 2 has already been registered to the DTCP device ID registerunit 10, or when the register of the device ID has been completed instep 518, the remaining processing of the authentication & key exchangeprocessing is performed with the receiver 2 to exchange the key (stepS19).

According to the present embodiment, when performing wirelesscommunication under the standard of Bluetooth, the RTT measurement isperformed to protect copyright of the contents. In this case, there arematters to be taken into consideration as a unique characteristic ofBluetooth.

(1) Sniff Mode

Bluetooth is often implemented to equipment which requires low powerconsumption such as a cellular phone and the other mobile devices.Therefore, Bluetooth standardizes various schemes for realizing lowpower consumption. One of the schemes is the sniff mode. Bluetooth is anetwork technique for performing one-to-many communication. A sourcenode in that network is called as a master, and the other branch nodesare called as slaves.

FIG. 6 is a diagram showing a connection relationship between the master30 and the slaves 31, and FIG. 7 is a diagram explaining the sniff mode.Each of the slaves 31 communicates with the master by wireless. Thesniff mode is a low power consumption mode unique to the slaves 31. Timeslots, which is also called as sniff slots, are provided for everyconstant interval, and transmission and reception of the packets arelimited in these time slots.

Each interval of scales in FIG. 7 expresses a transmission cycle of thepacket by the master 30 or the slaves 31. “M” expresses the transmissioncycle allocated to the master 30, and “S” expresses the transmissioncycle allocated to the slaves. The master 30 and the slaves 31 arepermitted to transmit the packets only within the sniff cycle T1determined in advance. FIG. 7 shows the sniff cycle T1 of a certainslave 1. Arrows of FIG. 7 become a transmission timing of the slave 31.

The slave 31 operated in the sniff mode receives the packets within thesniff cycle T1. When the address set to the packet header is not its ownaddress, the reception of the packet is suspended along the way, and theslave waits for the subsequent sniff cycle T1.

On the other hand, when the received packet is for its own, as long asthe transmission packet from the master 30 exists, the packets arereceived successively. At this time, in the time slot just after thetime slot in the sniff cycle T1 which has received the packet, it ispossible to transmit the packet to the master 30. When the time slot isnot within the sniff cycle, transmission and reception of the packetsare suspended in principle, thereby lowering power consumption.

In this way, the slaves 31 operating in the sniff mode communicate withthe master 30 only within the sniff cycle T1. Therefore, even if theslave 31 receives the packets for the RTT measurement from the master30, it is impossible to send back the response to the master 30 untilthe subsequent sniff cycle. Because of this, there is likelihood thatthe RTT value may become very large.

Therefore, according to the present embodiment, when performing the RTTmeasurement, the sniff cycle T1 is shortened, and response of the packetfor the RU measurement is quickly sent back.

The sniff cycle T1 is set in the Bluetooth interface units 16 and 26 inFIGS. 2 and 3. More specifically, the sniff cycle can be changed byusing an HCI (Host Controller Interface) command expressing API forsetting hardware of the Bluetooth device or a LMP (Link layer ManagementProtocol) command. For example, by issuing the LMP command(LMP_sniff_req) by the Bluetooth interface unit 16, it is possible tochange the value of the sniff cycle T1. Therefore, the Bluetoothinterface units 16 and 26 issue the above command to temporarily shortenthe sniff cycle T1 when performing the RTT measurement, therebycorrectly performing the RTT measurement.

(2) Polling Interval

On the other hand, in the cellular phone, which is the most populardevice that Bluetooth is implemented, voice communication is Inevitable,and it is necessary to maintain quality (QoS) of the network. Therefore,various schemes for improving QoS are implemented in the standard ofBluetooth. One of the schemes is polling. The polling gives a chance ofpacket transmission to the slaves 31 at least once during a constantperiod to obtain QoS more than a predetermined value. The cycle of thepolling is called as a polling interval.

FIG. 8 is a diagram explaining a polling interval. As shown in FIG. 8,time slots are allocated for the respective master 30 and slaves 31.Each slave 31 performs wireless communication with the master 30 onlywithin the polling cycle. If the polling interval T2 is long, when acertain slave 31 receives the packet for the RTT measurement, theresponse for the received packet has to wait until the subsequentpolling. Therefore, the waiting time becomes long, and as a result, theRUT measurement value becomes large.

According to the present embodiment, in order to decrease the RTTmeasurement value, the polling interval T2 is shortened at the RTTmeasuring time. The Bluetooth interface unit 16 of FIGS. 2 and 3performs modification of the polling interval T2. More specifically, themodification can be performed by using the HCI command expressing APIfor setting hardware of the Bluetooth device and the LMP command. Forexample, it is possible to modify the polling cycle by issuing the LMPcommand (LMP_quality_of_service) by the Bluetooth interface unit 16.

(3) Wireless Transmission Power

According to the standard of Bluetooth, there are some modes relating towireless transmission power intensity. That is, there are some modescalled as classes. Class 1 expresses that wireless radio field intensityis strong (even in a distance of 100 m, it is possible to communicatebetween the transmitter 1 and the receiver 2), and class 2 expressesthat wireless radio field intensity is weak (only within a distance of5-10 m, it is possible to communicate between the transmitter 1 and thereceiver 2). As a scheme of ensuring that the transmitter 1 and thereceiver 2 are located adjacent to each other, it is more desirable touse the radio field intensity of class 2 than that of class 1.

Accordingly, when performing transmission of contents for whichcopyright protection is necessary, it is desirable to use the radiofield intensity of class 2 between the transmitter 1 and the receiver 2to weaken the radio transmission power intensity.

(4) Master Slave Exchange

As shown in FIG. 6, each device for communicating by Bluetooth has arole called as the master 30 or the slave 31. The master 30 has acontrol function of the Bluetooth network (called as a Bluetoothpiconet). All the other devices connected to the Bluetooth network arethe slaves 31. The slaves of maximum seven devices can communicate withone master 30.

Control function for the master 30 is, for example, to allocate the timeslots for the respective slaves. In Bluetooth, the master 30 performsallocation of the time slots. In order to fulfill requirement fordecreasing the RTT measurement value as small as possible by sendingback the RTT response at once, it is desirable to provide configurationthat the receiver 2 for transmitting the RTT response becomes the master30. If the receiver 2 is the master 30, at a time of receiving the RTTcommand from the transmitter 1, the time slot can be controlled so as tosend back the response to the transmitter 1.

Generally, it is impossible to predict which of the transmitter 1 or thereceiver 2 becomes the master 30. Because of this, according to thestandard of Bluetooth, a scheme called as master slave exchange isprovided to exchange roles of the master 30 and the slaves 31. By usingthe scheme, the transmitter 1 can become the slave 31, and the receiver2 can become the master 30. On the contrary, the transmitter 1 canbecome the master 30, and the receiver 2 can become the slave 31.

By using this scheme, during the RTT measurement period, a device whichreceives the RTT command and sends back the response, for example, thereceiver 2, may be set to the master 30. Therefore, the receiver 2 canallocate the time slots for itself at a time point of receiving the RTTcommand, and can soon send back the response to transmitter 1. As aresult, the RTT value becomes small.

As described above, when performing the RTT measurement, it is desirableto change in advance the Bluetooth parameters such as (1) the sniffmode, (2) the polling interval T2; (3) the wireless transmission powerand (4) the master slave exchange. It is unnecessary to change all theparameters of the above (1) to (4). Even if only a portion of theparameters (1) to (4) is changed, it is possible to relatively preciselymeasure the RTT. Hereinafter, a timing of changing these parameters willbe described in detail below.

FIG. 9 is a sequence diagram showing a first example of a timing ofchanging the Bluetooth parameters.

In FIG. 9, after the authentication & key exchange processing is begun(step S21), the Bluetooth parameters for the RTT measurement are changed(step S22). Here, the Bluetooth parameters include at least one of theabove (1) to (4). More specifically, when the sniff cycle T1 is changed,the sniff cycle T1 is changed to be small so as to be able to quicklysend back the response to the transmitter 1. When the polling intervalT2 is changed, the polling interval T2 is changed to be small in thesame way. When the wireless transmission power is changed, the radiofield intensity is weakened so as to be able to transmit the packetsonly within a close range. When performing the master slave exchange,the receiver 2 for receiving the RTT packet is set to the master 30, andcontrol authority of the timing slots is given to the receiver 2.

Afterward, the RTT measurement is begun (step S23). When the RTTmeasurement is completed, the Bluetooth parameters are put back (stepS24). And then the remaining processing of the authentication & keyexchange processing is performed (step S25). When the authentication &key exchange processing is successful, AV application is started up, andthe AV contents for which copyright protection is necessary aretransmitted and received in the AV application (step S26).

FIG. 10 is a sequence diagram showing a second example of a timing ofchanging the Bluetooth parameters. Before beginning the authentication &key exchange processing, the Bluetooth parameters are changed for theRTT measurement (step S31). Subsequently, the authentication & keyexchange processing is performed (step S32). Along the way of theprocessing, the RTT measurement is performed (step S33). Subsequently,the remaining processing of the authentication & key exchange processingis performed (step S34). These steps S32 to S34 are performed in thesame procedure as that of FIGS. 4-5.

When the authentication & key exchange processing is successful, theBluetooth parameters changed in step S3.1 are put back (step S35). Andthen the AV application is started up, and the AV contents aretransmitted (step S36).

FIG. 11 is a sequence diagram showing a third example of a timing ofchanging the Bluetooth parameters. First of all, the AV application isstarted up (step S41). Subsequently, in the same way as step S22, theBluetooth parameters for the RTT measurement are changed (step S42).Subsequently, the commands of the AV application are transmitted andreceived between the transmitter 1 and the receiver 2 (step S43).

Subsequently, the authentication & key exchange processing is performed,and the RTT measurement is performed in the processing (step S44). Thestep S44 is performed in the same procedure as that of FIGS. 4-5. Whenthe authentication & key exchange processing is normally completed, theprocessing of the AV application including the transmission of the AVcontents is performed (step 545).

Subsequently, after the AV application is completed, the Bluetoothparameters are put back (step S47).

In FIGS. 9-11, the RTT measurement is performed along the way of theauthentication & key exchange processing based on the processingprocedure of FIGS. 4-5. However, the RTT measurement may be performedbefore or after the authentication & key exchange processing. Forexample, if the authentication & key exchange processing is performedafter the RTT measurement is performed, it is possible to permit thebeginning of the authentication & key exchange processing only when theRTT measurement is successful, and it is unnecessary to perform theauthentication & key exchange processing in vain. Furthermore, if theRTT measurement is performed after performing the authentication & keyexchange processing, it is possible to omit the RTT measurement when theauthentication & key exchange processing is not successful, and it isunnecessary to perform the RTT measurement in vain. In this case, forexample, when the authentication & key exchange processing issuccessful, but the RTT measurement is unsuccessful, the processingwhich does not permit the use of the encryption key obtained by theauthentication & key exchange processing is performed.

In FIGS. 9-11, the example of integrally performing the processing ofchanging the wireless parameters has been described. The timings ofchanging the wireless parameters may be individually set for eachparameter. That is, modification of a partial parameter may be performedat a timing of FIG. 9, and modification of the other parameter may beperformed at a timing of FIG. 11.

As described above, according to the present embodiment, when thecontents for which copyright protection is necessary is transmitted andreceived via the wireless network 3 of Bluetooth, before beginning theRTT measurement, in order to limit the range of transmission of thecontents, the Bluetooth parameters are changed to correctly perform theRTT measurement. Therefore, it is possible to transmit the contents onlywithin a range restricted physically such as inside-home. Accordingly,even when the wireless network 3 of Bluetooth is used, it is possible toprotect copyright of the contents. Especially, according to the presentembodiment, even when the transmitter 1 and the receiver 2 discretelycommunicate with each other by using the sniff mode or the pollinginterval unique to Bluetooth, it is possible to correctly measure theRTT, and to precisely and easily determine whether the transmission ofthe contents should be permitted.

At least a portion of functions performed by the above-mentionedtransmitter 1 and receiver 2 may be constituted by at least one ofhardware and software. When constituted by software, a program ofexecuting at least a portion of the functions performed by thetransmitter 1 and the receiver 2 is stored in a recording media such asa floppy disk or CD-ROM, and is loaded to a computer to execute itsprogram. The recording media is not limited to a portable media such amagnetic disk or an optical disk, but a fixed recording media such as ahard disk drive or a memory may be used to store the program.

The program of executing at least a portion of the functions performedby the transmitter 1 and the receiver 2 may be distributed via acommunication line such as Internet. The program may be distributed viaa wired line or a wireless line such as Internet at a state ofencrypting, modulating or compressing the program, or may be distributedat a state of being stored in the recording media.

1. A transmitter, comprising: a network interface unit connected to awireless network capable of transmitting contents for which copyrightprotection is necessary; an encryption processing unit configured toencrypt contents for which copyright protection is necessary; an RTTmeasuring unit configured to measure a round trip time after apredetermined packet is transmitted to a receiver, until a responsecorresponding to the predetermined packet is received; a communicationpermission determination unit configured to permit transmission of thecontents for which copyright protection is necessary when the round triptime measured by the RTT measuring unit is within a predetermined time;and a parameter modification unit configured to change parameters of thewireless network, before the RTT measuring unit measures the round triptime, the parameters improving accuracy of the round trip time when theRTT measuring unit measures the round trip time, wherein the wirelessnetwork is Bluetooth; and the parameter modification unit changes atleast one of a sniff interval expressing transmission and receptioninterval, a polling interval, transmission power and master-slaveexchange prescribed by a standard of Bluetooth as parameters.
 2. Thetransmitter according to claim 1, further comprising an authentication &key exchange unit configured to perform authentication & key exchangeprocessing with the receiver; wherein the parameter modification unitchanges the parameters to measure the round trip time when theauthentication & key exchange unit performs the authentication & keyexchange processing, and restores the parameters after the measurementof the round trip time is completed, before the authentication & keyexchange unit completes the authentication & key exchange processing. 3.The transmitter according to claim 1, further comprising anauthentication & key exchange unit configured to perform authentication& key exchange processing with the receiver; wherein the parametermodification unit changes the parameters to measure the round trip timewhen the authentication & key exchange unit performs the authentication& key exchange processing, and restores the parameters after theauthentication & key exchange unit completes the authentication & keyexchange processing.
 4. The transmitter according to claim 1, furthercomprising an authentication & key exchange unit configured to performauthentication & key exchange processing with the receiver; wherein theparameter modification unit changes the parameters to measure the roundtrip time before transmission of commands relating to contents for whichcopyright protection is necessary is begun, and restores the parametersafter transmission processing of contents for which copyright protectionis necessary is completed.
 5. The transmitter according to claim 1,wherein the parameter modification unit sets the sniff interval shorterthan a normal interval when the RTT measuring unit performs themeasurement.
 6. The transmitter according to claim 1, wherein theparameter modification unit sets the polling interval shorter than anormal interval when the RTT measuring unit performs the measurement. 7.The transmitter according to claim 1, wherein the parameter modificationunit sets a transmission power weaker than a normal power when the RTTmeasuring unit performs the measurement.
 8. The transmitter according toclaim 1, wherein the parameter modification unit reverses roles of amaster device and a slave device when the RTT measuring unit performsthe measurement.
 9. A receiver, comprising: a network interface unitconnected to a wireless network capable of receiving contents for whichcopyright protection is necessary; an encryption processing unitconfigured to decrypt contents for which copyright protection isnecessary; an RTT measuring unit configured to measure a round trip timeafter a predetermined packet is transmitted to a transmitter, until aresponse corresponding to the predetermined packet is received; acommunication permission determination unit configured to permitreception of contents for which copyright protection is necessary whenthe round trip time measured by the RTT measuring unit is within apredetermined time; and a parameter modification unit configured tochange parameters of the wireless network, before the RTT measuring unitmeasures the round trip time, the parameters improving accuracy of theround trip time when the RTT measuring unit measures the round triptime, wherein the wireless network is Bluetooth; and the parametermodification unit changes at least one of a sniff interval expressingtransmission and reception interval, a polling interval, transmissionpower and master-slave exchange prescribed by a standard of Bluetooth asparameters.
 10. The receiver according to claim 9, further comprising anauthentication & key exchange unit configured to perform theauthentication & key exchange processing with the transmitter, whereinthe parameter modification unit changes the parameters to measure theround trip time when the authentication & key exchange unit performs theauthentication & key exchange, and restores the parameters after themeasurement of the round trip time is completed, before theauthentication & key exchange unit completes the authentication & keyexchange processing.
 11. The receiver according to claim 9, furthercomprising an authentication & key exchange unit configured to performthe authentication & key exchange processing with the transmitter;wherein the parameter modification unit changes the parameters tomeasure the round trip time before the authentication & key exchangeunit begins the authentication & key exchange processing, and restoresthe parameters after the authentication & key exchange unit completesthe authentication & key exchange processing.
 12. The receiver accordingto claim 9, further comprising an authentication & key exchange unitconfigured to perform the authentication & key exchange processing withthe transmitter, wherein the parameter modification unit changes theparameters to measure the round trip time before beginning transmissionof commands relating to transmission of contents for which copyrightprotection is necessary, and restores the parameters after transmissionprocessing of contents for which copyright protection is necessary iscompleted.
 13. The receiver according to claim 9, wherein the parametermodification unit sets the sniff interval shorter than a normal intervalwhen the RTT measuring unit performs the measurement.
 14. The receiveraccording to claim 9, wherein the parameter modification unit sets thepolling interval shorter than a normal interval when the RTT measuringunit performs the measurement.
 15. The receiver according to claim 9,wherein the parameter modification unit sets a transmission power weakerthan a normal power when the RTT measuring unit performs themeasurement.
 16. The receiver according to claim 9, wherein theparameter modification unit reverses roles of a master device and aslave device when the RTT measuring unit performs the measurement.
 17. Anon-transitory computer readable recording medium for storing acommunication control program, the program, when executed, causes acomputer to perform a method comprising: measuring a round trip timeafter a predetermined packet is transmitted to the other communicationapparatus, until a response corresponding to the predetermined packet isreceived; permitting transmission or reception of contents for whichcopyright protection is necessary when the measured round trip time iswithin a predetermined time; transmitting or receiving the encryptedcontents via a wireless network when transmission or reception of thecontents is permitted; and changing parameters of the wireless network,before measuring the round trip time, the parameters improving accuracyof the round trip time when the RTT measuring unit measures the roundtrip time, wherein the wireless network is Bluetooth; and the changingparameter changes at least one of a sniff interval expressingtransmission and reception interval, a polling interval, transmissionpower and master-slave exchange prescribed by a standard of Bluetooth asparameters.